The human body was built to move. Therefore, you’ve got to find a way to make your figures move in whatever pose they’re in. But how do you do that? You’ve probably heard of the line of action. It’s a long line sketched in at the rough stage of the drawing to emphasize the fluidity of the pose. The line of the spine serves the same purpose.
Whichever way the spine is curving or bending is the same way the body generally moves. So use the line of the spine (indicated in red in the drawings here) to indicate this flow of action. When you sketch the line of the spine in your rough drawings, try to make it curve, and you’ll avoid making a stiff drawing.